Since a rate of a channel resistance in an on-resistance has been high in a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) with a trench type gate structure, a channel density has been increased by miniaturization etc. to thereby achieve reduction of the on-resistance. When reducing the on-resistance is achieved to some extent by increasing the channel density, next, reducing a resistance of a drift layer has been required.
MOSFET structures to achieve reduction of the resistance of the drift layer include a field plate structure (hereinafter also represented as an FP structure), a super junction structure (hereinafter also represented as an SJ structure), etc. Since a depletion layer formed in the drift layer can be extended more widely using any structure, an impurity concentration in the drift layer can be made high, and further, a high breakdown voltage can be obtained. In addition, when this kind of MOSFET is an n-channel type element, generally, an n-type impurity is contained in the drift layer, and a p-type impurity is contained in a base layer in which a channel is formed.
However, when an n-type impurity concentration contained in the drift layer becomes not less than a certain concentration, a mobility of the drift layer may decrease rapidly. Additionally, in this kind of MOSFET, the p-type impurity larger in amount than the n-type impurity is injected on a surface of the drift layer, and the base layer different from the drift layer in a conductivity type is formed on the surface of the drift layer. Hence, in the base layer, originally contained is substantially the same amount of n-type impurity as the amount of n-type impurity contained in the drift layer. Accordingly, in an example of the n-channel type MOSFET, there is a possibility that an n-type impurity concentration contained in the base layer affects a resistance of the channel formed in the base layer.